c# - 空引用异常和??运算符(operator)

Question

我在Asp.Net MVC应用程序中有一个窗体，该窗体在下一行（但仅在回发错误之后）返回Null引用异常：

<%= Html.TextArea("Description", Model.Evt.Description ?? "")%>

字符串Description确实为空，但我的期望是，如果Model.evt.Description为null，则TextArea中将显示“”。在最初加载表单时效果很好时，为什么要这样做呢？

唯一的区别是，从用于初始加载的新对象填充了ViewUserControl的类型，并在回发重新加载时填充了模型对象。

我想念什么？

堆栈跟踪如下：

[NullReferenceException: Object reference not set to an instance of an object.]
   System.Web.Mvc.HtmlHelper.GetModelStateValue(String key, Type destinationType) +63
   System.Web.Mvc.Html.TextAreaExtensions.TextAreaHelper(HtmlHelper htmlHelper, String name, Boolean useViewData, String value, IDictionary`2 implicitAttributes, IDictionary`2 explicitParameters, IDictionary`2 htmlAttributes) +261
   System.Web.Mvc.Html.TextAreaExtensions.TextArea(HtmlHelper htmlHelper, String name, String value, IDictionary`2 htmlAttributes) +213
   System.Web.Mvc.Html.TextAreaExtensions.TextArea(HtmlHelper htmlHelper, String name, String value) +61
   ASP.views_events_eventform_ascx.__Render__control1(HtmlTextWriter __w, Control parameterContainer) in c:\Documents and Settings\Richard Box\My Documents\Visual Studio 2008\Projects\EventsManager\EventsManager\Views\Events\EventForm.ascx:64
   System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) +256
   System.Web.UI.Control.RenderChildren(HtmlTextWriter writer) +19
   System.Web.UI.Control.Render(HtmlTextWriter writer) +10
   System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) +27
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter) +99
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer) +25
   System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) +134
   System.Web.UI.Control.RenderChildren(HtmlTextWriter writer) +19
   System.Web.UI.Page.Render(HtmlTextWriter writer) +29
   System.Web.Mvc.ViewPage.Render(HtmlTextWriter writer) +59
   System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) +27
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter) +99
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer) +25
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +1266

Answer 1

看起来Model或Evt为null，实际上不是Description字段。